The use of aerosol devices to administer drug or other therapeutically active compounds by inhalation therapy is common, particularly for the treatment of respiratory disorders, such as, asthma. Aerosol containers are charged with a self-propelling liquid composition comprising an aerosol propellant having medicament dispersed or dissolved therein and the container is equipped with a valve capable of discharging a metered amount of the self-propelling composition.
Metered dose valves which have been used on commercially available aerosol inhalers comprise a fixed metering chamber having a movable valve stem extending therethrough. When the valve stem is in its closed position, pressurized aerosol formulation in the aerosol container is free to pass into the metering chamber. As the valve stem is moved to its firing position, communication between the metering chamber and aerosol container is blocked thereby defining a fixed volume of pressurized aerosol formulation within the metering chamber. Further movement of the valve stem causes a discharge orifice in the valve stem to enter the metering chamber thereby allowing the contents of the chamber to be expelled through the valve stem under the influence of the aerosol propellant.
It has been found with some metered dose valves that there is variation from the target dose after a storage period e.g. of several hours. In particular, it has been observed that the drug delivered in the first shot after storage is below the target dose and a second shot, fired within a few seconds of the first shot, delivers an amount of drug above the target dose. It is postulated that the normal shaking of the aerosol container prior to firing the valve may not be sufficient to agitate the contents in the metering chamber which may contain sedimented or creamed drug deposited during the storage period. Thus, some of the sedimented or creamed drug is left behind in the metering chamber when the first dose is dispensed and the introduction of fresh formulation into the metering chamber after the first shot rinses and agitates the sedimented or creamed drug. When the second shot is fired the drug content of the dose is high.
In some metered dose valves there is a partially enclosed volume through which the pressurized aerosol formulation must pass before entering the metering chamber. In such pre-metering regions it is possible that drug may sediment or cream and deposit on surfaces of the valve and may not be re-suspended upon normal shaking of the aerosol container because of limited movement of the aerosol formulation in the partially enclosed volume. When a first dose is dispensed, introduction of fresh formulation into the pre-metering region during filling of the metering chamber may rinse the deposited drug from valve surfaces causing variation from the target concentration of drug.
It is an object of the present invention to provide a metered dose dispensing valve in which this problem is reduced.